Catalan President Artur Mas (C) joins hands with candidates for Junts Pel Si (Together For Yes) as they address supporters after polls closed in a regional parliamentary election in Barcelona, Spain, September 27, 2015.Separatists have won a clear majority of seats in Catalonia's parliament, an exit poll showed on Sunday, in an election that could set the region on a collision course with Spain's central government over independence. REUTERS/Sergio Perez BARCELONA (Reuters) - Separatists have won a clear majority of seats in Catalonia's parliament, preliminary official results showed on Sunday, in an election that set the region on a collision course with Spain's central government over independence

"Catalans have voted yes to independence," acting Catalan regional government head Artur Mas told supporters, with secessionist parties on track to secure 72 out of 135 seats in the powerful region of 7.5 million people that includes Barcelona.

The strong pro-independence showing dealt a blow to Spanish Prime Minister Mariano Rajoy, three months before a national election. His center-right government, which has opposed attempts to hold a referendum on secession, has called the separatist plan "a nonsense" and vowed to block it in court.

Spain's constitution does not allow any region to break away, so the prospect of a breakaway remains highly hypothetical.

The main secessionist group "Junts pel Si" (Together for Yes) was on track to secure 62 seats, while the smaller leftist CUP party would get another 10, according to official results with 97 percent of the votes counted.

They would jointly obtain 47.8 percent of the vote, on a record turnout of 78 percent -- a big boost to an independence campaign which has been losing support over the last two years.

Both had said before the vote that such a result would allow them to unilaterally declare independence within 18 months, under a plan that would see the new Catalan authorities approving their own constitution and building institutions like an army, central bank and judicial system.

Addressing supporters of Junts pel Si in central Barcelona, Mas said a "democratic mandate" now existed to move forward with independence.

"That gives us a great strength and strong legitimacy to keep on with this project," Mas told the exultant crowd, which chanted "in-inde-independencia" and waved secessionist flags.

Albert Llorent, a taxi driver from Barcelona who had come to celebrate, said the result was of historic proportions.

"What I think, what I feel, is that I belong to the best possible nation in the world. Long live Catalonia," he said.

Spain's Prime Minister Mariano Rajoy. Thomson Reuters CONSEQUENCES?

The vote in Catalonia, Spain's second most populous region, is widely expected to influence the course of the Spanish general election in December.

Spain's two dominant parties - the ruling People's Party and the opposition Socialists - lost tens of thousands of votes compared with the last vote in 2012, boding ill for their national ambitions, although the PP suffered a much deeper setback than its rival.

Anti-austerity Podemos also registered a disappointing score at 9 percent, sharply down from last May's nationwide regional and local elections.

Among parties opposed to independence, pro-market Ciudadanos, often cited as a national kingmaker, emerged as the only winner as it jumped to 18 percent of the vote.

Supporters of secessionist group Junts Pel Si (Together for Yes) react after polls closed in a regional parliamentary election in Barcelona, Spain, September 27, 2015. REUTERS/Sergio Perez Despite the separatist victory, analysts believe the most likely outcome of the election will be to force a dialogue between Catalan and Spanish authorities.

"Many have voted for Junts pel Si even if they don't favor secession because they saw the vote as a blank cartridge... and a way to gain a stronger position ahead of a negotiation," said Jose Pablo Ferrandiz from polling firm Metroscopia.

Opinion polls show a majority of Catalans would like to remain within Spain if the region was offered a more favorable tax regime and laws that better protect language and culture.

While investors do not see secession as an immediate material risk, financial markets may react negatively on Monday.

The gap between Spanish five-year bond yields and the higher yields of the Catalan equivalents has been hovering near its widest point in two years in the run-up to the vote.

(Additional reporting by the Madrid Newsroom; Editing by Mark Trevelyan)